The invention relates to a method for the calibration of a flow measurement in a flow system, and also to a flow system for carrying out the method.
It is barely possible to keep pace with the number of methods and applications which are now known for the measurement of flow, in other words for the determination of the amount of a fluid that is pumped through a flow system per unit of time.
Flow rates are measured for liquids, for example in injection systems for combustion engines, such as for motor vehicles, airplanes, or ships. Flow rates of crude oil through pipelines have to be measured and monitored continuously, for example for invoicing purposes. In the filling of containers, filling quantities have to be measured and metered, etc.
It is nowadays standard practice to make flow measurements of gaseous fluids, such as natural gas through a pipeline, or for invoicing purposes when refueling motor vehicles, with the aid of the most varied systems.
In this connection a large number of physical values and characteristics of the fluid or of the flow system are available for the determination of the amount flowing, through the measurement of which the amount of the fluid flowing through the flow system can be determined.
The parameter which is specifically used for the measurement of the amount flowing in any given case can depend on many factors. It can depend, on the one hand, on whether the fluid is liquid or gaseous, or even solid, such as for example fine abrasive sand flowing in a sandblasting device or a fine spray powder flowing in a thermal spraying apparatus. As used herein, the term “fluid” should be understood to include granular and powdered solids. Certain parameters are then suitable or not suitable from the outset. Thus, for example, in the determination of the flow of a gas, special efforts have to be made since, for example, the measurement of the volume flowing per unit of time through the flow system only makes limited statements about the amount of mass flowing, since with a given volume flow this depends very much on the temperature and the pressure of the gas, for example.
In the flowing transport of liquid materials, on the other hand, values such as the pressure and the temperature of the fluid often do not play a role, so that—at least when no real precision is demanded—a measurement of the volume flowing through the flow system per unit of time is completely adequate to draw conclusions on the mass of material which has actually flowed.
Thus, it is often possible and also appropriate to provide within a flow connection of the flow system simple sensors, such as impellers, floating bodies, aperture diaphragms for measuring in accordance with the differential pressure method, venturi nozzles (in the case of gases), and many other measuring devices well-known to the person averagely skilled in the art, so that the flowing fluid can interact directly with the corresponding measuring device (with the impeller for example), so that the flow rate is directly deducible, for example from the rotary speed of the impeller.
Thus, in simple cases, i.e. when no really strict demands are made on the precision of the flow measurement and/or when, for example, no hygienic or technically sensitive liquids have to be measured, such as blood, pharmaceutically or chemically ultra-pure liquids, or ultra-pure liquids in the semiconductor industry, for example slurries for the polishing of semiconductor wafers, a plurality of measuring systems and methods are available which are partly interchangeable at will and which completely fulfil the technical requirements in relation to precision, long-term stability and chemical or physical compatibility with the fluid to be measured.
Greater efforts have to be made, however, when the demands on the flow measurement increase. For example, during the measurement of liquids, temperature fluctuations or alterations in pressure can influence the measurement result, when precision of the results is required. Depending on the method of measurement, other parameters, such as the viscosity of the fluid for example, which can change in the course of the measuring process, can unacceptably influence the results.
It is also of the greatest significance in some applications that the measurement does not take place invasively, if at all possible. I.e. it is often essential that the measuring device as such does not come into direct physical contact with the fluid to be measured. This can be significant, for example, when the fluid is not compatible with the measuring body, for example chemically or physically, for example because the fluid is very aggressive chemically as in the case of a strong acid or lye, or is physically aggressive, such as for example the above-mentioned slurry, which acts highly abrasively and could destroy a measuring body, such as a floating body or an impeller, in a very short time.
However, not only can the measuring apparatus be affected negatively by the fluid to be measured; vice versa, in some cases the measuring apparatus can also have negative repercussions on the fluid to be measured.
Thus, in the case of sensitive liquids, such as blood or ultra-pure pharmaceutical or chemical products, impurities have to be avoided at all costs. In the case of a mechanical bearing for example, in which an impeller is journalled for the measurement of the flow rate, this can lead to contamination of the fluid in the flow system in the form of bearing lubricants or mechanically abraded particles.
For example, blood is a liquid which also reacts to mechanical influences extremely sensitively. If the flow system is a ventricle support system for the support of a human or animal blood circulation, in other words a flow system made of a blood pump, cannulae and feed lines, which forms a bypass for the heart during an operation for example, or supports a weakened heart in a long-term application, the quantity of blood flowing is a very critical system parameter, which has to be monitored constantly and possibly sensitively readjusted by suitably controlling the blood pump.
In a flow system of this kind the measurement of the flow rate of the blood is a particularly critical challenge with regard to various aspects.
If constrictions and/or a locally heavy mechanical load arises in the ventricle support system, for example a compression of the blood between an impeller of a flow measuring component and a wall of the flow connection, then a destruction or crushing of the red blood corpuscles can result, or adsorption and/or accretion of blood on the flow measuring body, for example on an impeller for flow measurement, can take place and, in the worst case, can even lead to severe agglutination of the blood, which can lead to serious damage to the patient, such as thromboses, vascular occlusions and even to infarct, in the worst case to the death of the patient.
For this reason in medical systems such as these—however, also in purely technical systems, for example for the semi-conductor industry where, as described above, very high purity is required—additional invasive measuring apparatuses are preferably not provided in the flow system in order to monitor the flow rates of the fluid to be pumped.